In the development of gas turbine engines, it is important to determine the amount of vibration of the rotating blades. From vibration measurements, stresses induced in the blades may be determined. Action can then be taken to avoid stresses which are high enough to cause damage to the blades.
A technique for characterising blade vibration is “blade tip timing” (BTT) in which non-contact timing probes (e.g. capacitance or optical probes), typically mounted on the engine casing, are used to measure the time at which a blade passes each probe. This time is compared with the time at which the blade would have passed the probe if it had been undergoing no vibration. This is termed the “expected arrival time” and can be calculated from the rotational position of the particular blade on the rotor in conjunction with a “once per revolution” (OPR) signal which provides information about the position of the rotor. The OPR signal is derived from the time at which an indicator on the rotor passes a reference sensor, and its use is well known in the art.
The difference between the expected arrival time and the actual arrival time can be multiplied by the blade tip velocity to give the displacement of the blade from its expected position. Thus BTT data from a particular probe effectively measures blade tip displacement at the probe.
Advantageously, the tip timing method does not require a telemetry system since the probes are mounted on the casing. However, because the sampling rate of the probes is determined by the rotational frequency of the rotor, it is often below the Nyquist frequency for the vibrations of interest. Thus each probe undersamples the vibrations, leading to problems such as aliasing. A further problem with BTT data is that it is often intrinsically noisy due to probe movement caused by mounting restrictions and casing thickness. Nonetheless, with a plurality of timing probes, it is possible, in principle, to perform useful vibration analysis that can be converted into blade stresses.